When compared to commercial inorganic fertilizers, all manure is low in nutrient levels. It releases nutrients slowly and only when the soil is warm and moist enough for microorganisms to break down the components. While each batch of manure will vary in nutrient content, horse and cow manure have more similarities than differences.
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Manure isn't made up of feces and urine alone. By the time it reaches your garden, manure may include animal feed, bedding, soil, plant matter and many other materials. The amount of nutrients in the manure will be directly affected by the quality and composition of the animal's diet, as well as its size and physical condition. You would have to analyze a sample to know the exact breakdown of its nutrient content. For most home gardeners, that level of detail isn't necessary.
When nitrogen, phosphorus and potassium levels are analyzed for cow manure, a distinction must be made between dairy cows and beef cattle, primarily due to their different diets. Within the dairy cow group, calves, heifers, lactating cows, dry cows and veal calves will show variations. The size of the cow also affects the nutrient levels in manure from beef cattle. For cow manure, in general, nitrogen will vary from 0.5 to 2.0, phosphorus from 0.2 to 0.9, and potassium from 0.5 to 1.5.
The variation in nutrient composition between batches of horse manure depends on what the horses were fed. Manure from horses eating all forage has a slightly lower nutrient content than those fed a 50-50 mix of grain and forage. In horse manure, nitrogen tends to range from 0.5 to 2.5, phosphorus from 0.3 to 2.5, and potassium from 0.5 to 3.0.
Fresh or Composted Manure
Nutrient levels are highest in fresh manure, but it may contain weed seeds, salts and disease-causing bacteria and viruses such as Salmonella and E. coli. To avoid contamination of edible plants, do not apply fresh manure on crops that grow near the ground and are eaten raw. Whenever fresh manure is used, wait three to four weeks after applying before you plant seedlings. Composting manure removes the issue of weeds seeds, salts, bacteria and viruses. You can apply composted manure at planting time or use it to side-dress growing plants.
Either or Both
Because the nutrient value is relatively small for both horse and cow manure, it's best to consider them soil conditioners rather than fertilizers. As a soil conditioner, either form of manure protects against erosion, helps the soil absorb water, improves drainage, creates better soil structure and enhances microbe activity. Whichever manure is more readily available is the best one to use. If you have access to both horse and cow manure, put them both to work in your garden.